Lithium ion secondary batteries using electrolytic solutions have higher operating potential, smaller size, and higher energy density than aqueous solution-based secondary batteries and therefore are widely used as power supplies for cellular phones, notebook computers, and the like. But, as rapid development of portable electronic equipment and use in electric cars have been achieved in recent years, a further improvement in energy density has been required.
Examples of methods for improving energy density include a method using an active material having large capacity, a method of increasing the operating potential of a battery, and methods of improving charge and discharge efficiency, cycle life, and the like. Among these, the method of increasing the operating potential of a battery can provide an assembled battery having a smaller number of series than a conventional assembled battery and therefore is means effective for the size reduction and weight reduction of a battery module used in an electric car or the like.
As the positive electrode active materials of lithium ion secondary batteries, 4 V class active materials (average operating potential=3.6 to 3.8 V: versus lithium potential) such as lithium cobaltate and lithium manganate are known. On the other hand, for example, it is known that compounds obtained by replacing the Mn of spinel type lithium manganate by Ni, Co, Fe, Cu, Cr, or the like can be used as 5 V class active materials. These compounds have an average operating potential of 4.6 V or more versus lithium potential.
For example, LiNi0.5Mn1.5O4 has a capacity of 130 mAh/g or more and an average operating potential of 4.6 V or more versus Li metal. Therefore, it is expected as a material having high energy density. Further, spinel type lithium manganese oxides have advantages such as comprising a three-dimensional lithium diffusion path, having better thermodynamic stability than other compounds, and being easily synthesized.
In addition, polyolefin microporous films typified by polyethylene and polypropylene are widely used in lithium ion secondary batteries. In the polyolefin microporous film, when about 120 to 170° C. is reached, a shutdown function in which the separator melts to close the pores of the separator to stop the battery function is exhibited, and the safety of the battery is maintained.
As an example of a separator using a polyolefin microporous film, Patent Literature 1 discloses disposing a microporous film layer made of polypropylene on the surface of a separator opposed to a positive electrode.
Patent Literature 2 discloses a separator provided with a layer comprising at least one material from polypropylene, a polyaramid, a polyamideimide, and a polyimide.
Patent Literature 3 discloses a separator in which a fibrous flame-retardant compound such as glass fibers and aramid fibers is dispersed in a polyolefinic resin in order to improve the thermal stability of the separator.
In addition, separators containing cellulose such as paper and nonwoven fabrics are also used. Separators comprising cellulose and separators comprising glass fibers have excellent heat resistance and high load characteristics.
For example, Patent Literature 4 discloses a separator comprising cellulose fibers in a lithium ion secondary battery by defining the porosity, pore diameter, and pore specific surface area of the separator.
In Patent Literature 5, a cellulose separator is used by using, as a positive electrode active material, a spinel-based manganese complex metal oxide having an about 130° C. higher oxygen release temperature than a lithium cobalt complex metal oxide and a low need for a shutdown function.
Patent Literature 6 discloses a lithium ion secondary battery using a separator comprising glass fibers.